On the mechanism of ring metallation of aromatic compounds. Metallation of thiophen by lithium and by lithium dihydroarylides

Abstract

Thiophen is metallated by lithium metal or lithium dihydroarylides in the 2-position. Product yields and magnetic titration results indicate that the transformation is a two electron process. The process becomes one-electron when the reaction is carried out in the presence of 1,1-diphenylethylene or α-methylstyrene. Paramagnetic decay in solutions of various lithium dihydroarylides and thiophen is of the first order. The log k₁ values correlate with the π-energy change of the reaction ArH^{[graphic omitted]}→ ArH + e^–. A mechanism for the metallation of thiophen by lithium dihydroarylides involving an unstable thiophen radical-anion intermediate is suggested. It is also suggested that metallation by conventional lithiating agents takes place through the intermediacy of a radical-anion.